As integrated circuit devices become increasingly highly-integrated, and patterned features descend to 65 nm and beyond, it is becoming increasingly desirable to form low-k (dielectric constant k<3.0) or ultra-low-k (dielectric constant ≦2.5) dielectric layers on a semiconductor wafer, for example as an interlayer dielectric between different layers of metallization, in order to attain desired capacitance values. (To simplify, in the present document the expression “low-k dielectric” is intended to cover both low-k and ultra-low-k dielectrics, unless the context indicates otherwise).
In order to produce a dielectric layer having a suitably low dielectric constant, it is known to introduce small pores (typically the size of one or several nanometers) into a material which already has a low dielectric constant, such as poly(methylsilsesquioxane), p-MSQ. As a first step, a film is created which is an inorganic matrix of p-MSQ containing a porogen, such as acrylate. Then, in a second step, the porogen is expelled, leaving a nanoporous film having a low dielectric constant. The same type of two-step process is used for producing mesoporous or nanoporous low-k dielectric layers of other materials.
Different techniques have been proposed for expelling the porogen in order to produce the desired low-k dielectric layer. Initial proposals involved applying heat to the semiconductor wafer, in a thermal annealing step. However, the elevated temperatures required for thermal annealing, and the duration of the heating periods, often produced undesirable effects on other components of integrated circuit devices being formed on the wafer (e.g. on Cu metal lines already present on the wafer). Accordingly, other proposals have suggested that the initial non-porous layer of dielectric material could be rendered porous by curing it, notably by application of ultraviolet radiation (UV). This approach is illustrated schematically in FIG. 1, in which the thickness of the dielectric layer is greatly exaggerated compared to the thickness of the other wafer layers. The UV curing step generally involves heating the semiconductor wafer, but for a much shorter time than would be necessary for a thermal annealing process alone (e.g. 1/20 of the time needed for thermal annealing).
As illustrated in FIG. 1A, a layer 2 of dielectric material is formed on a wafer 1. This layer 2 consists of a matrix having distributed within it a pore-generating material (porogen). The wafer 1 is placed on a chuck 5 which is heated while UV radiation is applied to the layer of dielectric material 2 (see FIG. 1B). As UV curing takes place, the porogen is expelled from the layer of dielectric material 2, as indicated by the curved arrows in FIG. 1B. After UV curing, the dielectric material has tiny voids, or pores, 3 distributed throughout it, as indicated in FIG. 1C. As a result, the dielectric layer 2 now has a reduced value of dielectric constant
Different materials have been proposed for use in forming porous low-k dielectric layers on a semiconductor wafer. However, the porosity of the low-k dielectric material generally results in low mechanical strength (generally a Young's modulus value of much less than 10 GPa, and low hardness) compared with traditional SiO2 dielectric layers which are relatively tough (having Young's modulus in the range of around 46 to 92 GPa, depending upon the technique used for formation of the SiO2 layer). In the case of methylsilsesquioxane (MSQ), the expulsion of porogens (in order to achieve low-k) reduces the mechanical strength from around 14 GPa to below 6 GPa. In the case of the MSQ-based material Zirkon™ LK2000, produced by RHEM, the expulsion of porogens reduces the mechanical strength from around 10 GPa to around 5 GPa.
When examining the properties of low-k dielectric materials a correlation has been found between lower dielectric constant and reduced Young's modulus. This effect is illustrated in the graph of FIG. 2.
Moreover, this problem of low mechanical strength affects layers made of various low-k dielectric materials. It is especially acute for spin-on deposited dielectric materials, because of a relatively lower degree of cross-linking than CVD deposited films. Organic polymers, in particular, have low mechanical stability.
It is believed that a Young's modulus of at least 6 GPa is required in order for a layer of material to withstand integration into conventional fabrication processes. In fact, the low-k dielectric material can often have such low mechanical strength that it becomes impossible to apply subsequent process steps, notably conventional packaging steps, or even copper chemical mechanical planarization (CMP). Typically, the low-k dielectric layer may peel off during Cu CMP, or crack during packaging. Accordingly, such low-k dielectric materials can only be used if special techniques are employed for the subsequent processing steps, which increase the cost and complexity of the manufacturing process.
It is desirable to be able to produce a low-k dielectric layer on a semiconductor wafer while ensuring that this layer maintains a sufficient degree of mechanical strength to enable it to withstand subsequent conventional process stages.
The present invention provides a fabrication method for fabricating a semiconductor wafer comprising a low-k dielectric layer, in accordance with the appended claims.
The present invention further provides a semiconductor wafer comprising a low-k dielectric layer, in accordance with the appended claims.
Certain presently-preferred embodiments of the invention will now be described, by way of example, with reference to the drawings, of which: